Add tests for almasru, fix imports all tests

marc_tools/tests/test_marc_extractor.py

 import pytest
-import marc_extractor
+import marc_tools.marc_extractor as marc_extractor
 
 
 class TestConstructSingleCommand:

sru/tests/error.xml

+<?xml version="1.0" encoding="UTF-8"?><searchRetrieveResponse xmlns="http://www.loc.gov/zing/srw/" xmlns:diag="http://www.loc.gov/zing/srw/diagnostic/">
+  <version>1.2</version>
+  <diagnostics>
+    <diag:diagnostic>
+      <diag:uri>200801</diag:uri>
+      <diag:message>Catalog search has encountered an error</diag:message>
+    </diag:diagnostic>
+  </diagnostics>
+</searchRetrieveResponse>
\ No newline at end of file

sru/tests/test_almasru.py

+import pytest
+import sru.almasru as almasru
+
+retriever = almasru.RecordRetriever('obv-at-oenb', '43ACC_ONB', 'marcxml')
+
+
+class TestRecordRetriever:
+
+    def test_erroneous_xml(self):
+        with pytest.raises(almasru.SruException):
+            retriever.by_mms_id('MMS-ID')
+